The most common mode of rejuvenating a previously sealed concrete surface, now dull and weathered, is to apply one or more applications of a solvent-borne acrylic lacquer containing acrylic solids dispersed in either an aromatic or mineral spirits solvent carrier.
Recently, due to requirements for reduced VOC emissions, flammable solvents such as acetone, tertiary-butyl acetate, para-chlorobenzenetrifluoride, and dimethyl carbonate have replaced most or all of the traditional petroleum solvents rendering the acrylic lacquers more difficult to apply during warm seasons. The evaporation rates of these VOC exempt solvents are much higher, leading to increased rates of drying. A phenomenon known as “cob webbing” can take place as more of the solvent evaporates before the resin has had an opportunity to completely flow out over the concrete surface leaving behind thin strands of cotton-like web over the surface of the sealant.
In an attempt to avoid the problems associated with solvent-based lacquers, water-based compositions have been developed that comprise emulsion polymerized acrylic copolymers and styrenated acrylic copolymers. These emulsions, when applied over non-porous substrates such as metal, typically produce a high gloss; however, when applied over porous surfaces such as concrete and masonry, only produce, at best, a satin finish or modest gloss. This reduced gloss is directly attributable to the water-like viscosity of the acrylic emulsions and their poor hold-out at the surface of the porous concrete or masonry. In addition, acrylic emulsions exhibit mediocre wetting characteristics due to the high surface tension of the continuous aqueous phase relative to the surface energy at the concrete interface. This phenomenon of poor wetting is exacerbated by the presence of highly absorptive pigments and metal stearates, which are used to produce decorative stamped concrete finishes. Commonly, stearate and pigments are left in the crevices and joints created within the geometric pattern left behind by a stamp imprint. Thus, acrylic emulsions do not provide high gloss, depth of finish, or good color rejuvenation.
Furthermore, the repeated resealing of concrete and masonry surfaces with solvent-based acrylic and styrenated acrylate solution resins initially leads to a high gloss finish, but frequently also leads to excessive film build up. Such resealing is often done annually to maintain its desired appearance. As a consequence of this increased film thickness, the moisture vapor transmission rate, known traditionally as the sealant's breathability to water vapor, is impeded drastically. This frequently results in the sealant turning milky white (blushing), down glossing, blistering, flaking and peeling. To remedy this problem, all the coats of sealant must be removed. Removal can be done mechanically or chemically, but only chemical removal is effective to maintain the appearance of stamped or decorative surfaces. The chemicals used to remove the sealant, however, are typically harsh, high in VOC, and will destroy the surrounding vegetation.